1. Field of the Invention
The present invention relates to a novel phosphonium salt, an organically modified layered silicate organophilized with the phosphonium salt and a composition thereof. More precisely, the present invention relates to a novel phosphonium salt having both of superior interfacial chemical characteristics and superior thermal stability, an organically modified layered silicate organophilized with the phosphonium salt and a composition thereof.
The present invention relates to the field of materials of polymer composites which can exhibit their performance by addition of the aforementioned phosphonium salt, the aforementioned organically modified layered silicate or a composition thereof, including packaging materials of foods, pharmaceutical preparations etc., flame retardant materials used as members of buildings, electric equipments, automobiles etc., viscosity modifiers, additives in cosmetics etc., coating agents, fibers and so forth.
2. Description of the Background
Onium compounds having a long-chain alkyl group are conventionally known to function as cationic surfactants. Among these, quaternary ammonium salts are widely used as cationic surfactants in practice. For example, such quaternary ammonium salts are widely used as surfactants such as those used in hair conditioners and softeners, organophilizing agents for exchanging inorganic cations existing between layers of layered silicates such as bentonite and montmorillonite for quaternary ammonium salts.
It is known that a molecular structure of a surfactant significantly affects surface activity and molecular aggregate structure, and this is systematically explained on the basis of hydrophilic/lipophilic balance of the surfactant. That is, a quaternary ammonium salt having one long-chain alkyl group has hydrophilicity stronger than lipophilicity and therefore forms micelles in water. On the other hand, a quaternary ammonium salt having two long-chain alkyl groups has balanced hydrophilicity and lipophilicity and therefore forms lamellar liquid crystals or bilayer membrane vesicles in water.
Further, it is also known that a difference in molecular structure of a cationic surfactant significantly affects dispersing characteristic of an “organically modified layered silicate” obtained by exchanging inorganic cations in a layered silicate for the cationic surfactant. Therefore, examples of use of a quaternary ammonium salt having a branched alkyl group in a layered silicate as an organophilizing agent have been reported so far, which direct attentions to the difference in the molecular structure (refer to Japanese Patent Laid-open Publication (Kokai) No. 9-309720 (page 2, claim 1, page 3, paragraph [0006] to page 4, paragraph [0012]), for example). In this document, it has been reported that an organically modified layered silicate organophilized with a quaternary ammonium salt having a branched alkyl group exhibited more favorable dispersibility in an organic solvent and thermoplastic resin compared with an organically modified layered silicate organophilized with a quaternary ammonium salt having a straight alkyl group.
However, the aforementioned organically modified layered silicate using a quaternary ammonium salt as an organophilizing agent has a problem that when it is melt-kneaded with a thermoplastic resin at a temperature of 200° C. or higher, the quaternary ammonium salt is decomposed into a tertiary amine and olefin by the Hofmann elimination (or Hofmann degradation) and hence cannot function as an organophilizing agent. The aforementioned organically modified layered silicate organophilized with a quaternary ammonium salt also has a problem that when it is dispersed in an organic solvent and used as grease, the quaternary ammonium salt is thermally decomposed under a high temperature condition of 200° C. or higher (condition of heating site such as bearing of motor) and hence cannot be used under such a condition.
Further, the aforementioned problem of the thermal decomposition of quaternary ammonium salt is also observed in use of quaternary ammonium in the field of semiconductor nanoparticle synthesis by the hot soap method, for example. That is, since the reaction in the hot soap method is performed at a high temperature of 250 to 300° C., the quaternary ammonium salt is thermally decomposed under such a condition and therefore cannot be used as cationic protective colloid.
Quaternary phosphonium salts can be mentioned as oniums having superior thermal stability as compared with quaternary ammonium salts, and in particular, tetraalkylphosphoniums have superior thermal stability. Functions of tetraalkylphosphoniums having such a characteristic have been investigated from various viewpoints, and it has been revealed that they can be applied to phase transfer catalysts, antibacterial agents, thermotropic liquid crystal (A. W. Herriott, D. Picker, “Journal of the American Chemical Society”, 97, 2345-2349 (1975); A. Kanazawa et al., “Journal of Polymer Science”, Part A: Polymer Chemistry, 31, 3003-3011 (1993); and D. J. Abdallah et al., “Journal of the American Chemical Society”, 122, 3053-3062 (2000), for example) and so forth.
However, in all of these documents, straight tetraalkylphosphonium salts and ammonium salts are compared as surfactants, and none of them discloses dispersibility attributable to their molecular structures. Furthermore, none of these documents discloses applicability of tetraalkylphosphonium salts as heat resistant surfactants or organophilizing agents, either.
Therefore, development of cationic surfactants having surface activity and thermal stability comparable or superior to those of quaternary ammonium salts and organically modified layered silicates having superior thermal stability and dispersibility has been desired.